It is becoming increasingly common to provide electronic controls for mechanical devices, in fact, electronic controls can greatly enhance the effectiveness of various mechanical devices. For example, electronic detectors can measure the angle at which the steering wheels of a vehicle are turned and use the information to turn a vehicle's headlights to eliminate the road in the direction of the turn. One problem that does exist for such controls, however, is the maintenance of a home position to which a device, such as a headlight beam control, should return while the electronic control is not being called upon. In the case of electronically controlled headlights, the headlight should return to a home position in which they are aimed directly down a roadway when the vehicle wheels are neither turning to the right nor the left. Mechanical devices exist for finding a home position, but mechanical devices are subject to wear and parts that are in continuous use, as are adjustable headlights, may suffer wear and failure before other portions of a vehicle. One method of electronically detecting a home position for a moveable element is to provide a magnet on the moveable element and place an analog-type Hall effect detector at the home position. The output from the analog Hall effect detector is directed to a microcomputer and will enable the microcomputer to determine which direction the moveable member should move to return to its home position. Such Hall effect analog-type detectors are expensive, costing as much as four dollars each, and where numerous controls are needed for a complex device such as an automobile, the cost of providing analog-type Hall effect detectors in various control devices can be prohibitive. Switch-type Hall effect detectors, on the other hand, cost only about fifteen cents each.
It would be desirable to provide an electronic control that would return a moveable device to a home position and would rely upon electronic controls, such as a switch-type Hall effect detector, however, such switch-type detectors have only two states, activated by a magnetic field and deactivated. Furthermore, switch-type Hall effect detectors become activated when in the presence of a magnetic field that exceeds a given threshold. Where a magnet generates a field which significantly exceed the threshold for the switch-type Hall effect detector, the detector will remain in the activated position as a moveable member moves through the activation range. In the absence of further controls, a microcomputer relying upon input from a switch-type Hall effect detector is unable to accurately determine a home position within the actuation range of the switch. Furthermore, once the moveable member falls outside of an actuation range for a switch-type Hall effect detector, the detector provides no information as to which direction a moveable member must go if it is to return to its home position.
One obvious method of providing a home position is to provide fixed information as to the location of the home position directly into a microcomputer. For example, a headlight control that includes a motor that rotates a threaded shaft can receive input from a counter that counts the rotations of the shaft and records the direction of rotation thereby enabling the microcomputer to add or subtract rotations to measure movement of the moveable member away from its home position in either direction. It has been found, however, over long periods of time a microcomputer can lose an accurate memory of the home position. This could occur as a result of errors developing in the home position code stored within the microcomputer, or as a result of mechanical errors such as slippage of a follower with respect to a threaded member. Over a significant period of time, the home position for an adjustable headlight may become so seriously lost that the headlights are chronically directed to one side of the road or the other.